An ultralow temperature cold trap

Abstract

The application relates to the field of low-temperature cold trap filtering technology and discloses an ultralow-temperature cold trap which comprises a shell and an upper cover, a refrigeration assembly and a filtering assembly are detachably arranged in the shell, the filtering assembly is sleeved on the refrigeration assembly, the filtering assembly comprises a filtering column, at least two filtering discs are movably arranged on the filtering column, each filtering disc comprises a main bonding disc and a secondary extruding disc, the application is characterized in that the filtering discs are movably connected, the relative space size of different layers of filtering discs can be adjusted according to the different amounts of impurities adsorbed by the activated carbon in the filtering discs, the adsorption amount of the activated carbon in different layers is adjusted, the activated carbon in the multiple layers can be completely adsorbed at the same time in a small interval, the phenomenon that the bottom layer is completely adsorbed but the top layer still has a large amount of activated carbon which is not completely adsorbed is avoided, the activated carbon is not completely adsorbed, the whole filtering assembly is directly replaced, and the service time and performance of the filtering assembly are reduced.

Description

Technical Field

[0001] This invention relates to the field of cryogenic cold trap filtration technology, specifically to an ultra-low temperature cold trap. Background Technology

[0002] Cryogenic cold trap filtration is a physical separation technology that utilizes extremely low temperatures (typically below -80°C, reaching liquid nitrogen temperatures as low as -196°C) to condense and capture volatile impurities (such as water vapor, organic solvent vapor, oil vapor, etc.) in gas or vapor streams. Its core is a condensing surface (cold trap) maintained at an ultra-low temperature. When the target gas stream passes through, condensable impurities solidify or liquefy due to the sudden temperature drop, thus being efficiently captured and removed from the gas stream. This process purifies the gas (e.g., protecting vacuum systems, purifying carrier gas) or recovers specific volatile substances.

[0003] Condensed impurities are generally adsorbed by activated carbon. Since gas is mostly introduced from bottom to top, activated carbon needs to be placed in a position that allows easy contact with air if it is to come into contact with the gas. Nowadays, most filters are divided into disc type and cylindrical type. The disc is usually inverted in the cold trap, so activated carbon is mostly installed on the disc by adhesive. However, after the activated carbon adsorbs a lot of impurities, its own weight will increase. Moreover, at low temperatures, the performance of most viscous substances will decrease. Therefore, under the combined effect of multiple factors, there is a risk that the activated carbon will fall off. If the activated carbon falls to the bottom of the cold trap, it will not only be unable to adsorb, but may also block the air inlet and other slots, and it will not be easy to remove.

[0004] Because the gas flows from bottom to top, in a disc-type filter, the lower discs will have a greater adsorption capacity than the upper discs. Therefore, even after the activated carbon in the lower layer has completely adsorbed the gas, some of the activated carbon in the upper layer will still remain unadsorbed. However, since the activated carbon in the lower layer cannot adsorb the gas, the performance of the disc filter is greatly reduced, so a new disc needs to be replaced. At this time, the unadsorbed activated carbon in the upper layer will be wasted. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides an ultra-low temperature cold trap that has the advantage of preventing activated carbon from falling off inside the cold trap.

[0006] To achieve the above objectives, the present invention provides the following technical solution: including a shell and a top cover, wherein a refrigeration component and a filter component are detachably disposed inside the shell, wherein the filter component is fitted onto the refrigeration component;

[0007] The filter assembly includes a filter column, on which at least two filter discs are movably mounted. Each filter disc includes a main bonding disc and a secondary extrusion disc. A number of activated carbons are bonded to one end face of the main bonding disc, and the secondary extrusion disc is hollowed out.

[0008] The main bonding disc and the secondary extrusion disc are attached together, and both slide on the filter column. At least two symmetrically arranged sliding cavities are provided inside the filter column.

[0009] A slider is slidably disposed inside the sliding cavity. The slider is fixed together with the main adhesive plate. A main spring is installed between the bottom of the slider and the bottom end face of the sliding cavity. A contact switch is provided on the top of the slider.

[0010] The slider has a notch, and the filter column is also movably equipped with an annular synchronous ring. The synchronous ring is fixed to the auxiliary extrusion plate by a connecting block. The synchronous ring is located in the notch, and an auxiliary spring is installed between the lower end face of the synchronous ring and the bottom end face of the notch.

[0011] All the filter discs have the same radius for the main bonding disc and the secondary extrusion disc. The main bonding disc has at least two flow ports on its end face near the filter column. The edge of the main bonding disc extends obliquely away from the filter column, forming an inverted shape. The secondary extrusion disc is the same as the main bonding disc, and its oblique edge also extends away from the filter column, forming a circular groove. The circular groove is located at the oblique edge of the main bonding disc. The activated carbon is disposed between the main bonding disc and the secondary extrusion disc.

[0012] The outer shell is also provided with an air inlet pipe and a vent plate. The air inlet pipe is located at the bottom of the outer shell and connects the outside to the internal cavity of the outer shell. The vent plate is provided with several drainage plates, which are installed at an angle.

[0013] The refrigeration assembly includes a refrigeration unit, which is fixed to the bottom center of the housing by bolts. A refrigeration column is provided on the refrigeration unit, and a filter column is fitted onto the refrigeration column.

[0014] Preferably, the radius of the filter disc decreases from top to bottom.

[0015] Preferably, the main bonding disc of the filter disc is corrugated, with its corrugated surface tilted downwards and abutting against the secondary extrusion disc. A number of activated carbon particles are adhered to the corrugated surface of the main bonding disc near the secondary extrusion disc.

[0016] Preferably, the top cover is detachably mounted on the outer casing, and the top cover is provided with an air outlet and an alarm block, with the air outlet located at the center of the top cover.

[0017] Compared with the prior art, the present invention provides an ultra-low temperature cold trap, which has the following beneficial effects:

[0018] 1. This ultra-low temperature cold trap, through the setting of the movable filter plates, can adaptively adjust the size of the relative space of the filter plates according to the different amounts of impurities adsorbed by the activated carbon in different layers of filter plates. This adjusts the adsorption amount of activated carbon in different layers, so that the activated carbon in multiple layers can be completely adsorbed at the same time within a short interval. This avoids the phenomenon that the bottom layer is completely adsorbed, but there is still a lot of activated carbon in the top layer that has not been completely adsorbed. If the activated carbon is not completely adsorbed, the entire filter component will need to be replaced directly, which will reduce its service life and performance.

[0019] 2. This ultra-low temperature cold trap, through the setting of the edge of the main bonding plate and the secondary extrusion plate, allows air to come into contact with the activated carbon through the secondary extrusion plate and guides the air to flow upward from the flow port. On the other hand, the secondary extrusion plate can prevent the activated carbon from falling to the bottom of the shell after falling off, making it difficult to remove. In particular, the circular groove of the secondary extrusion plate further prevents the activated carbon from slipping off the edge of the secondary extrusion plate.

[0020] 3. In this ultra-low temperature cold trap, after the activated carbon adsorbs a large amount of impurities, its own weight will increase. Moreover, at low temperatures, the viscosity of sticky substances will decrease, so there is a risk that the activated carbon may fall off. When a small amount falls off, the elasticity of the secondary spring can support the secondary extrusion plate and the main bonding plate to hold the activated carbon. At this time, the synchronization ring is still abutting against the slider. When a large amount falls off, the secondary extrusion plate and the main bonding plate will move away from each other, thereby pressing down the secondary spring through the synchronization ring, causing the synchronization ring to disengage from the inner wall of the abutment notch. Therefore, the contact switch will be triggered, and the alarm block will be activated to remind the staff that a large amount of activated carbon has fallen off, and the filter assembly needs to be replaced and the fallen activated carbon needs to be re-bonded. Attached Figure Description

[0021] Figure 1 This is a three-dimensional structural diagram of the present invention;

[0022] Figure 2 This is a schematic diagram of the half-section structure of the present invention;

[0023] Figure 3 For the present invention Figure 2 Enlarged structural diagram at point A;

[0024] Figure 4 For the present invention Figure 2 Enlarged structural diagram at point B;

[0025] Figure 5 This is a schematic diagram of the filter component structure of the present invention;

[0026] Figure 6 This is a schematic diagram of the filter disc structure of the present invention;

[0027] Figure 7 This is a schematic diagram of a half-section of the filter disc of the present invention;

[0028] Figure 8 This is a schematic diagram of the gas flow direction structure of the present invention;

[0029] Figure 9 This is a schematic diagram of the second type of structure of the filter component of the present invention;

[0030] Figure 10 This is a schematic diagram of the second type of filter disc structure of the present invention.

[0031] In the diagram: 10. Outer shell; 11. Inlet pipe; 12. Vent plate; 121. Drain plate; 20. Refrigeration unit; 201. Refrigeration column; 30. Top cover; 301. Outlet; 302. Alarm block; 40. Filter column; 401. Slide cavity; 41. Main bonding plate; 411. Flow port; 412. Slider; 413. Main spring; 414. Notch; 42. Secondary extrusion plate; 421. Synchronization ring; 422. Secondary spring. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0033] like Figure 1-7 As shown, the device includes a housing 10 and a top cover 30. A refrigeration component and a filter component are detachably installed inside the housing 10, with the filter component mounted on the refrigeration component. The housing 10 also includes an air inlet pipe 11 and a vent plate 12. The air inlet pipe 11 is located at the bottom of the housing 10, connecting the outside to the internal cavity of the housing 10. The air inlet pipe 11 can also be installed on the side wall of the housing 10, but it must be installed at the bottom of the side wall. The vent plate 12 is equipped with several guide plates 121, which are installed at an angle to guide the air to be filtered. The gas enters through the inlet pipe 11 and exits through the top cover 30. The chamber inside the outer shell 10 is cooled by the cooling components. The cooling temperature inside the ultra-low temperature cold trap can reach -105℃, which can condense gases with boiling points or freezing points far below the cold trap temperature, such as water vapor, diffusion pump oil vapor, and most organic solvents, thus obtaining pure gas. The condensed impurities are adsorbed by the filter components, allowing the cold trap to continue working. Then, the filtered gas is guided through the guide plate 121 in the vent plate 12 and collected in the middle of the top cover 30 before being extracted.

[0034] The refrigeration assembly includes a refrigerator 20, which is bolted to the center of the bottom of the outer casing 10. A refrigeration column 201 is mounted on the refrigerator 20, and a filter column 40 is fitted onto the refrigeration column 201. The refrigerator 20 is a Stirling refrigerator, and its working principle is based on the Stirling cycle, a closed-loop gas circulation process. It cycles through isothermal compression, isochoric cooling, isothermal expansion, and isochoric heating. Through this cycle, the Stirling refrigerator can transfer heat from a low-temperature region to a high-temperature region, thus achieving a cooling effect. After cooling, the heat diffuses outwards through the refrigeration column 201, reducing the temperature inside the outer casing 10. Because it is diffusion-type cooling, the temperature drop is lower at locations farther from the refrigeration column 201, resulting in a higher temperature at locations farther from the refrigeration column 201 than at locations closer to it. Figure 8 As shown, most of the gas flows out from the inside of the filter assembly, increasing the gas temperature drop and improving the gas condensation and filtration effect.

[0035] The top cover 30 is detachably mounted on the outer casing 10. The top cover 30 is provided with an air outlet 301 and an alarm block 302. The air outlet 301 is located at the center of the top cover 30. The air outlet 301 on the top cover 30 is connected to an external vacuum pump to extract the filtered gas inside the outer casing 10. The alarm block 302 can be used to detect the amount of activated carbon falling off the filter assembly. If too much activated carbon falls off, the alarm block 302 will alert the staff that the filter assembly needs to be replaced.

[0036] The filter assembly includes a filter column 40, which can be square or circular. At least two filter discs are movably mounted on the filter column 40. Each filter disc includes a main bonding disc 41 and a secondary extrusion disc 42. A number of activated carbons are bonded to one end face of the main bonding disc 41. The secondary extrusion disc 42 is hollow, and the activated carbons are bonded to the bottom end face of the main bonding disc 41 for adsorbing water vapor and other impurities.

[0037] The main bonding disc 41 and the secondary extrusion disc 42 are attached together and both slide on the filter column 40. At least two symmetrically arranged sliding cavities 401 are provided inside the filter column 40. A slider 412 is slidably arranged inside each sliding cavity 401. The slider 412 is fixed to the main bonding disc 41. A main spring 413 is installed between the bottom of the slider 412 and the bottom end face of the sliding cavity 401. A contact switch is provided on the top of the slider 412, and the contact switch is electrically connected to the alarm block 302. When the activated carbon adsorbs a large amount of impurities, its own weight increases, and at low temperatures, the viscosity of the sticky substance decreases, so there is a risk of the activated carbon falling off. When the fall is minimal, the elasticity of the secondary spring 422 supports the secondary extrusion disc 42 and the main bonding disc 41 in holding the activated carbon in place. When activated carbon falls, the synchronizing ring 421 is still abutting against the slider 412. When a lot of activated carbon falls, the auxiliary extrusion plate 42 and the main bonding plate 41 will move away from each other. This will cause the synchronizing ring 421 to press down on the auxiliary spring 422, causing the synchronizing ring 421 to disengage from the inner wall of the abutment notch 414. Therefore, the contact switch will be triggered, and the alarm block 302 will be activated to remind the staff that a large amount of activated carbon has fallen and the filter assembly needs to be replaced. The fallen activated carbon needs to be re-bonded. The slider 412 has a notch 414. The filter column 40 also has a movably arranged annular synchronizing ring 421. The synchronizing ring 421 is fixed together with the auxiliary extrusion plate 42 by a connecting block. The synchronizing ring 421 is located in the notch 414, and the auxiliary spring 422 is installed between the lower end face of the synchronizing ring 421 and the bottom end face of the notch 414.

[0038] When the activated carbon in the filter assembly adsorbs a large amount of water vapor and other impurities, its weight increases significantly. For example, if the bottom filter disc adsorbs a lot of impurities, it will cause the slider 412 to compress the main spring 413 and move downwards. The space between this layer's filter disc and the bottom will decrease, reducing the amount of air entering. At the same time, the distance between this layer and the layer above will increase, increasing the space between them and thus increasing the air content. The activated carbon in the filter disc of the upper layer can adsorb a large amount of impurities. When the activated carbon in the upper layer adsorbs a lot of impurities, the filter disc will also descend. Similarly, the space between the layers above will increase, and so on. Therefore, by setting up the movable filter discs, the relative space of the filter discs can be adjusted according to the different amounts of impurities adsorbed by the activated carbon in different layers. This adjusts the adsorption capacity of the activated carbon in different layers, allowing the activated carbon in multiple layers to be completely adsorbed simultaneously within a short interval. This avoids the situation where the bottom layer is completely adsorbed, but the top layer still has a lot of activated carbon that has not been completely adsorbed. If the activated carbon is not completely adsorbed, the entire filter assembly needs to be replaced, reducing its service life and performance.

[0039] All filter discs have the same radius for the main bonding disc 41 and the auxiliary extrusion disc 42. The main bonding disc 41 has at least two flow ports 411 on its end face near the filter column 40. The edge of the main bonding disc 41 extends obliquely away from the filter column 40, forming an inverted shape. The auxiliary extrusion disc 42 is identical to the main bonding disc 41, and its oblique edge also extends away from the filter column 40, forming an annular groove. This annular groove is located at the oblique edge of the main bonding disc 41. Activated carbon is disposed between the main bonding disc 41 and the auxiliary extrusion disc 42. The main bonding plate 41 near the flow port 411 can also be provided with several through holes for air circulation. The arrangement of the main bonding plate 41 edge and the secondary extrusion plate 42 allows air to come into contact with the activated carbon through the secondary extrusion plate 42 and guides the air to flow upward from the flow port 411. On the other hand, the secondary extrusion plate 42 can prevent the activated carbon from falling to the bottom of the outer shell 10 after it falls off, making it difficult to remove. In particular, the circular groove of the secondary extrusion plate 42 further prevents the activated carbon from slipping off the edge of the secondary extrusion plate 42.

[0040] Working principle: When in use, the refrigeration unit 20 can be started to refrigerate. When the temperature reaches the set temperature (105℃), the gas to be filtered can be introduced through the air inlet pipe 11. The gas will be gradually pushed upward and then connected to the vacuum pump through the air outlet 301 to extract the gas. The gas will flow from bottom to top inside the outer shell 10. Impurities in the gas will condense at low temperature and become liquid or solid floating matter. After the impurities come into contact with the activated carbon on the filter plate, they will be adsorbed, achieving the effect of removing impurities.

[0041] After the activated carbon on the filter disc adsorbs a large amount of impurities, its own weight increases, which causes the slider 412 to compress the main spring 413 and move downward. The space between the current filter disc and the next filter disc will decrease, and the amount of air entering will also decrease. At the same time, the distance between this layer and the previous layer will increase, and the space between this layer and the previous layer will increase, so the air content will increase. The activated carbon in the filter disc of the previous layer can adsorb a large amount of impurities. When the activated carbon in the previous layer adsorbs a large amount of impurities, the filter disc will also descend. At this time, the space between the next layer and the previous layer will increase, and so on, so that the activated carbon in each filter disc can be fully utilized.

[0042] However, the weight of a single activated carbon unit increases after absorbing a large amount of impurities, and the lower temperature causes the sticky substances to reduce performance, resulting in activated carbon shedding. The shed activated carbon falls onto the secondary extrusion plate 42. When only a small amount of activated carbon sheds, the secondary spring 422 remains compressed, and the elastic force it generates causes the synchronization ring 421 to abut against the end face of the notch 414. When a large amount of activated carbon sheds, it presses down on the secondary extrusion plate 42, causing the synchronization ring 421 to further compress the secondary spring 422, causing the synchronization ring 421 to detach and abut against the notch 414. Then, the contact switch on the synchronization ring 421 disconnects and contacts, triggering the alarm block 302 to start. The alarm is then activated by a light or bell to alert the staff that a large amount of activated carbon has shed, and under airflow conditions, the activated carbon may fall to the bottom of the outer casing 10, so it needs to be replaced. Example 2

[0043] like Figure 9 As shown, based on Embodiment 1, the filter discs are of different sizes, and the radius of the filter discs decreases from top to bottom. The uppermost filter disc can contact the inner wall of the outer casing 10, allowing the gas to pass through the filter disc completely and come into contact with the activated carbon, so that all the gas can be filtered. Since the size of each filter disc is different, the weight is also different, so the elasticity of the spring parts on each filter disc is also different, matching the weight of the filter disc and not affecting the movement of the filter disc. Example 3

[0044] As shown in Figure 10, based on Embodiment 1, the main bonding disk 41 of the filter disk can also be corrugated, with its corrugated surface tilted downwards and abutting against the secondary extrusion disk 42. A number of activated carbons are bonded to the corrugated surface of the main bonding disk 41 near the secondary extrusion disk 42, and the surface area of ​​the corrugated surface is significantly larger than that of the flat surface, so that it can bond more activated carbons, thereby increasing its service life and performance.

[0045] In summary, this cryogenic cold trap, through the movable filter discs, can adaptively adjust the relative space between different layers of filter discs according to the varying amounts of impurities adsorbed by the activated carbon. This regulates the adsorption capacity of the activated carbon in different layers, ensuring that the activated carbon in multiple layers can be completely adsorbed simultaneously within short intervals. This avoids the situation where the bottom layer is completely adsorbed, but the top layer still has a significant amount of unadsorbed activated carbon, requiring the replacement of the entire filter assembly due to incomplete adsorption, which reduces its service life and performance. Furthermore, the arrangement of the main bonding disc 41 edge and the secondary extrusion disc 42 allows air to contact the activated carbon through the secondary extrusion disc 42 and also guides the airflow upwards from the flow port 411. Additionally, the secondary extrusion disc 42 prevents activated carbon from falling to the bottom of the outer casing 10 after detachment, making it difficult to remove. The activated carbon is further prevented from slipping off the edge of the auxiliary extrusion plate 42, especially at the annular groove of the auxiliary extrusion plate 42. After the activated carbon adsorbs a lot of impurities, its own weight will increase, and the viscosity of the sticky substance will decrease at low temperature. Therefore, there is a risk that the activated carbon will fall off. When a small amount of carbon falls off, the elasticity of the auxiliary spring 422 can support the auxiliary extrusion plate 42 and the main bonding plate 41 to hold the activated carbon. At this time, the synchronization ring 421 is still abutting against the slider 412. When a large amount of carbon falls off, the auxiliary extrusion plate 42 and the main bonding plate 41 will move away from each other. As a result, the synchronization ring 421 will press down on the auxiliary spring 422, causing the synchronization ring 421 to disengage from the inner wall of the abutment notch 414. Therefore, the contact switch will be triggered, and the alarm block 302 will be activated to remind the staff that a large amount of activated carbon has fallen off and the filter assembly needs to be replaced and the fallen activated carbon needs to be re-bonded.

[0046] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0047] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A cryogenic cold trap, comprising a shell (10) and a top cover (30), characterized in that: The housing (10) may also be detachably equipped with a refrigeration component and a filter component, wherein the filter component is fitted onto the refrigeration component; The filter assembly includes a filter column (40), on which at least two filter discs are movably mounted. Each filter disc includes a main bonding disc (41) and a secondary extrusion disc (42). A number of activated carbons are bonded to one end face of the main bonding disc (41), and the secondary extrusion disc (42) is hollowed out. The main bonding disc (41) and the secondary extrusion disc (42) are attached together, and both slide on the filter column (40). At least two symmetrically arranged sliding cavities (401) are provided in the filter column (40). A slider (412) is slidably disposed inside the sliding cavity (401). The slider (412) is fixed together with the main adhesive plate (41). A main spring (413) is installed between the bottom of the slider (412) and the bottom end face of the sliding cavity (401). A contact switch is provided on the top of the slider (412). The slider (412) has a notch (414) inside, and a ring-shaped synchronous ring (421) is movably arranged inside the filter column (40). The synchronous ring (421) is fixed together with the auxiliary extrusion plate (42) through a connecting block. The synchronous ring (421) is located inside the notch (414), and an auxiliary spring (422) is installed between the lower end face of the synchronous ring (421) and the bottom end face of the notch (414). The main bonding disc (41) and the auxiliary extrusion disc (42) of all the filter discs have the same radius. The main bonding disc (41) has at least two flow ports (411) on one end face near the filter column (40). The edge of the main bonding disc (41) extends obliquely away from the filter column (40) and is in an inverted shape. The auxiliary extrusion disc (42) is consistent with the main bonding disc (41). Its oblique edge also extends away from the filter column (40) to form a circular groove. The circular groove is located at the oblique edge of the main bonding disc (41). The activated carbon is disposed between the main bonding disc (41) and the auxiliary extrusion disc (42). The outer shell (10) is also provided with an air inlet pipe (11) and a ventilation plate (12). The air inlet pipe (11) is located at the bottom of the outer shell (10) and connects the outside to the internal cavity of the outer shell (10). The ventilation plate (12) is provided with a number of drainage plates (121), and the drainage plates (121) are installed at an angle. The refrigeration assembly includes a refrigeration unit (20), which is fixed to the bottom center of the outer casing (10) by bolts. A refrigeration column (201) is provided on the refrigeration unit (20), and a filter column (40) is fitted on the refrigeration column (201).

2. The cryogenic cold trap according to claim 1, characterized in that: The radius of the filter disc decreases from top to bottom.

3. The cryogenic cold trap according to claim 1, characterized in that: The main bonding plate (41) of the filter disc is corrugated, and its corrugated surface is tilted downwards and abuts against the secondary extrusion plate (42). A number of activated carbons are adhered to the corrugated surface of the main bonding plate (41) near the secondary extrusion plate (42).

4. The cryogenic cold trap according to claim 1, characterized in that: The upper cover (30) is detachably mounted on the outer shell (10). The upper cover (30) is provided with an air outlet (301) and an alarm block (302). The air outlet (301) is located at the center of the upper cover (30).

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